Machine type communication (MTC) is considered as one of the major driving forces of a future generation of cellular communications. Likely application scenarios for MTC feature numerous low-cost machine-type devices connecting to the network. The data packet size of MTC is usually smaller than that of human-to-human cellular communications. Because of the smaller data packet sizes, iterative types of channel codes such as turbo codes or low density parity check (LDPC) codes would provide less coding gain with MTC than is usual with the longer human-to-human packet sizes. To make the matter worse, some types of MTC devices, like meter reading for utilities, are often installed in a basement or other area with poor signal penetration, which requires a superb link budget to overcome the deep penetration loss.
Coverage enhancements can be achieved in several ways. One approach is to use CDMA-like signals that have lower peak to average ratio, less control overhead, etc. A second approach is to use more antennas, either at the transmitter with beamforming or to obtain transmit diversity gain, or at the receiver to obtain aperture or receive diversity gain. A third approach is repetition, which has been used in 3GPP LTE, for the reason that LTE is an OFDMA system and there is no compelling reason to overhaul that fundamental just for the sake of coverage. Note that transmission time interval (TU) bundling enhancements, as described in Y. Yuan, et al, “LTE-Advanced coverage enhancements,” IEEE Comm. Mag, October, 2014, pp. 153-159, as one type of repetition, are already specified in Rel-11 LTE for uplink Voice over IP (VoIP) and data traffic carried on physical uplink shared channel (PUSCH). In Rel-12 and Rel-13 LTE, repetition is believed to the most effective technique to achieve good coverage of narrow-band MTC and has been extended to many other physical channels, for example, primary broadcast channel (PBCH), physical downlink shared channel (PDSCH), enhanced physical downlink control channel (EPDCCH), etc.
Network coding has attracted attention as an academic research topic. Its most promising use scenarios include relay, mesh networks, and device to device (D2D) communications.
Network coding takes advantage of the broadcast nature of wireless communications, and can make use of not-directly targeted transmission(s) to improve the redundancy of transmissions in a coordinated manner.
Significant impact on standards is expected if network coding is to be adopted in LTE. Since the channel coding would remain largely unchanged in LTE/LTE-A, network coding, which inevitably affects the channel codes, has not been studied in 3GPP LTE.